Epoxy resins are characterized by the presence of a three-membered cyclic ether group commonly referred to as an epoxy group, 1,2-epoxide, or oxirane. The most widely used epoxy resins are diglycidyl ethers of bisphenol A, derived from bisphenol A and epichlorohydrin. They are most frequently cured with anhydrides, aliphatic amines, or polyamides, depending on desired properties. Epoxy resins have been known to convey outstanding performance characteristics. However, various multifunctional resins, including epoxy cresol novolac resins (ECN) and polynuclear phenol-glycidyl ether-derived resins have been developed to further improve high temperature performance and other select properties.
Epoxy resins have played a very important role in the advancement of the information industry. A variety of epoxy resins, especially the o-cresol epoxy novolac resins, for example, have been widely used for the encapsulation of semiconductor devices and as adhesives in the microelectronic industry. Typically, liquid epoxy resins are synthesized by a two-step process in which an excess of epichlorohydrin is reacted with bisphenol A in the presence of at least a stoichiometric quantity of an alkaline catalyst, such as aqueous solutions of sodium hydroxide. The first step of the epoxy resin synthesis involves the formation of an intermediate, which is the dichlorodydrin of bisphenol A, and the second step involves a further reacion via dehydrohalogenation of the intermdeiate, again, with a stoichiometric quantity of alkali.
A number of improvements have been proposed in the prior art to improve the epoxy resin synthesis process. In U.S. Pat. No. 5,028,686, it was disclosed a concurrent addition process for preparing high purity epoxy resins in which epoxy resins which are relatively low in total bound halide are prepared by concurrently and continuously adding a mixture of (1) a mixture of an epihalohydrin, a compound containing an average of more than one group reactive with a vicinal epoxide group and a solvent and (2) an aqueous or organic solution of an alkali or alkaline earth metal hydroxide; to a mixture of epihalohydrin and a solvent.
U.S. Pat. No. 4,954,603 disclosed a process for making fire-retardant epoxy resins by reacting a trifunctional epoxy compound with halogenated bisphenol A in the presence of a catalyst, which comprised sodium hydroxide in a molar ratio of 2.85 to 1 relative to the trifunctional epoxy compound.
European Patent 579301 disclosed a process for producing 4,4'-biphenol skeleton-containing epoxy resins, by reacting 4,4'-biphenol with a epihalohydrin in a reaction medium of glycol monoethers. During the reaction, an alkali metal hydroxide was gradually added to the reaction mixture. The total amount of alkali metal hydroxide added was between 0.8 and 2.0 moles per mole of phenol groups.
European Patent 396203 disclosed an epoxy resin encapsulation composition comprising a tetrakisglycidyl ether of an .alpha.,.alpha.,.omega.,.omega.,-tetrakis(hydroxyphenyl)C.sub.4 -C.sub.14 alkane. The tetrakisglycidyl ether was produced by reacting an appropriate tetraphenol with a halohydrin in the presence of an alkali metal hydroxide.
All the above mentioned patents involved, or at least claimed, certain improvement over the conventional methods. However, all of them still share a common characteristic of the conventional methods in that all the claimed processes still involved the addition of a strong base, which typically contained an alkali metal hydroxide in an aqueous solution, or in a mixture of water and an alcohol solvent (such as isopropanol or butanediol, into the reaction mixture, which typically contained a polyphenol (bi-, tri-, or tetraphenol) and epihalohydrin. The amount of alkali metal hydroxide required is at least a stoichiometric quantity of the phenol groups (i.e., every mole of phenol group would require at least one mole of alkali metal hydroxide in a one-to-one quantitative substitution reaction).
One of the disadvantages of the conventional processes in preparing epoxy resins is that, because the addition of the strong base of alkali metal hydroxide is exothermic, it must be gradually added, as reported in all the references mentioned above. Furthermore, the addition of the alkali metal hydroxide solution introduces water into an otherwise organic system. This causes the reaction to be conducted in a non-homogeneous multi-phase condition; and typically, it also requires the reaction to be conducted under an azeotropic condition. After the reaction, the removal of the high-boiling point water or alcohol also involves a tedious process; it requires a set of relatively complicated post-reaction equipment and is time-consuming. Thus, it is desirable to develop alternative processes for making epoxy resins for use in the microelectronic industry which involve simplified procedure and require reduced reaction time.